The present invention relates to a composition and method for the treatment of architectural lead and, in particular, to a composition and method for the surface treatment of architectural lead using a patination oil having a particular formulation.
Traditionally, the appearance of architectural lead ranged from rough sand cast to a smooth but matt dark grey rolled finish, both of which would be fixed by traditional lead working techniques and allowed to weather naturally to a stable patina. During natural weathering, there is an initial stage where basic lead carbonate may be washed from the surface and form a white stain onto any adjacent material. Modern building design is such that run-off may take place from vertical surfaces such as mansards, often without a gutter underneath and directly on to masonry or brickwork. This change in design philosophy has also been accentuated by the advent of lead clad steel and lead coated chipboard. The change in appearance of new lead is also much more marked than in the past, due to the clean, very shiny surface produced by the latest generation of lead rolling mills. The lead sheet can have a brightly coloured surface which is due to interference colours produced by thin oxide films. The difference in appearance between old and new lead, where an old building is being renovated, is thus accentuated.
A patination oil was developed several years ago in response to the run-off from lead sheet staining buildings. This so-called patination oil is, in fact, a coating based on an alkyd resin, a silica flatting agent and other components in a dilute solution in white spirit. It forms a thin film on the surface of the lead which is just permeable enough to allow the slow formation of a stable basic lead carbonate patina underneath it. Over a period of months, it degrades in the sunlight and weathers to leave an even-coloured surface. Problems have been reported due to the silica flatting agent settling out prior to use, to form a thick sticky sludge. Furthermore, the existing formulations of patination oil do not have a significant effect on the oxide films formed on the surface of new lead, but tend to protect the oxide films. This can considerably extend the period during which the colouration discussed above is present. The coloured surface accentuates every imperfection and may lead to an architect rejecting the material, or giving a lower degree of market acceptance for the material.
We have now developed a modified patination oil for architectural lead which removes or modifies the oxide film to eliminate the objectionable colouration and leave a matt surface of a pleasing grey colour.